Potency standards for coagulation factors VIII (Antihemophilic Factor) and IX are needed to control the manufacture of these licensed products and to permit clinical assessment of their pharmacokinetics in patients. CBER has historically provided working standards for both products. In addition, it is desirable to establish standards that are recognized internationally in order to harmonize potency definitions and manufacturing requirements. A new U.S. standard for Factor IX has been established and adopted by the European Pharmacopoeia (EP) and World Health Organization (WHO). The standard was selected from among four intermediate- and high-purity factor IX preparations from four U.S. manufacturers. The selection was based on studies conducted at CBER that established the parallelism of dose-response curves for the standard and test materials; linearity of dose-response over a wide range of concentrations; inter-assay and intra-assay variability; physical integrity; and short-term stability. The selected factor IX standard was filled into 25,000 glass vials, each containing approximately 11 IU. The factor IX standard was then calibrated in a multicenter, international study involving 36 laboratories and assigned a value of 10.7 IU. The standard was accepted formally by the EP and the Expert Committee on Biological Standardization of the WHO in October, 1996. The standard is now available through these organizations and from CBER. Six candidates for a new U.S. factor VIII standard, derived from four U.S. manufacturers and including both plasma-derived and recombinant preparations, were initially evaluated by CBER. Each candidate was assessed for the same criteria indicated above and for consistency of results between two commonly used potency assays: the one-stage plasma assay and the chromogenic assay. Two candidates having satisfactory properties were selected for further evaluation in a multicenter, international study involving 18 laboratories, which was completed in early 1997. One candidate material was superior with respect to its unbiased, equivalent results in the one-stage and chromogenic assays, but had been lyophilized to an unacceptably high moisture content. This would likely have an adverse effect on stability if uncorrected. No reason for the high residual moisture has been identified, so an additional pilot fill is currently planned. In early 1997, the contractor who was to perform the filling and lyophilization of this standard informed CBER that changes in its technical staff made it impossible to continue with the project. An alternative form with the requisite capacity is currently being sought.